1. Field of the Invention
This invention generally relates to a data receiving system preferably used for digital mobile communications, and more particularly to a data receiving system capable of accurately detecting a control channel (FACCH) with reduced processing or computation amount.
2. Prior Art
In the mobile communication systems transmission/reception of various radio channels (including so-called control channels CCH as well as speech/data channels TCH) is essential for the "establishment of calling" and realization of "speech communications" and "data communications." One format example for radio channels used in the mobile communications is FIG. 8 which shows a data format applicable to the GSM system, while FIG. 9 shows another data format applicable to the PHS system.
For example, during "speech communications", there is a possibility that a mobile station shifts from the present cell to a neighboring cell. In such a cases a base station performs the processing for causing the speech channel to carry a so-called control channel FACCH thereon (hereinafter, this processing is referred to as "steal") and for transmitting a handover message to the mobile station.
In this manner, the control channel (FACCH) is an important channel in the start-up stage of the processing, for example for preventing the reception quality of the mobile station from being undesirably deteriorated (or preventing disconnecting of the call). Hence, failure in detecting the control channel (FACCH) will possibly result in a fatal failure in the continuation of the communications.
In view of the above, to increase the detection accuracy of control channel (FACCH), the following detecting method (Unexamined Japanese Patent application No. HEI 5-259960) has been already proposed. An operation of this conventional channel detecting method will be explained in greater detail with reference to the flow chart of FIG. 10.
It is supposed that the control channel (FACCH) and speech channel (UCH) are coded with different coding rates (R1 and R2) in a transmitter. When a receiver receives a data block coded in this manners the following processing is performed.
Step 1. A total of j nodes are decoded in accordance with the Viterbi algorithm as though the received data block itself or part of the same were a control channel (FACCH) block;
Step 2. The smallest accumulated metric m1=(mj).sub.min is determined after decoding of the j nodes. A threshold value T1 and a further threshold value T2 are predetermined by simulations, so as to obtain a minimum error probability;
Step 3. The value m1 is then compared with the threshold value T1;
Step 4. If m1&lt;T1 ("No" in the judgement), this implies that the data received can be definitely considered to be the control channel (FACCH) data, and therefore the control channel (FACCH) data block is decoded;
Step 5. The value of m1 is stored when m1&gt;T1 ("Yes" in the judgement) and the received data block is decoded as though it were a speech (UCH) data block. In this cases decoding can be executed with nodes k in accordance with the Viterbi algorithm;
Step 6. A new smallest value m2 of the accumulated metric m.sub.k is obtained, i.e. m2=(m.sub.k).sub.min ;
Step 7. A difference (m1-m2) is compared with the second threshold T2;
Step 8. If (m1-m2)&lt;T2 ("No" in the judgement), the data block is considered to be a control channel (FACCH) block and is decoded as control channel (FACCH) block; and
Step 9. If (m1-m2)&gt;T2 ("Yes" in the judgement), the data block is considered to be a speech channel (UCH) block and is decoded as speech channel (FACCH) block.
In this manner, according to the conventional data receiving system, the decoding processing is performed according to the Viterbi algorithm as through the received data were a control channel (FACCH) data. Then, using the smallest accumulated metric, the control channel (FACCH) is accurately detected and decoded.
However, this conventional data receiving system is disadvantageous in that the processing or computation amount is greatly increased because the decoding processing of the received data needs to be performed as control channel (FACCH) data in accordance with Viterbi algorithm in response to every decoding timing signal.